Internationally, telecommunications is undergoing major rapid change brought about by regulatory changes, increasingly open markets and technological advances in integrated circuits, optical devices, and computer systems. The convergence and the integration of these technologies, coupled with the driving factors of faster transmission speeds, lower signal levels, and denser circuit boards has made managing signals in electronic and communication switching systems critical.
These driving factors have placed a greater emphasis on managing problems relating to signal integrity, timing distribution, timing jitter, signal distribution, noise, asynchronism, and cross-talk. In long-haul transmission domain optical amplifiers, together with wavelength-division multiplexed have revolutionised high speed data transmission by providing flexible and cost-effective means of amplifying and processing of signals almost entirely in the optical domain independent of data rate and protocols.
Impairments suffered by timing signals play a critical role in electronic systems. They limit the dynamic range of an analogue-to-digital converter, the throughput of a digital bus, affect the behaviour of digital synchronisers, influence the bit error ratio of a communications link, and determine the sensitivity and selectivity of radio receivers. Timing impairments are the result of random noise and systematic disturbances within electronic devices and interconnections.
Electronically derived timing signals suffer from an additional inherency constraint; the limits of electronic system frequency response is predicated on the internal parasitic capacitances developed as an artifact of the functional underpinnings of active semiconductor integrated circuit devices. The familiar P-N junction which forms the basis of active device fabrication, whether expressed in terms of majority or minority carrier devices, nevertheless inherently compels a parasitic capacitance be coupled into the elemental circuit model.
Thus, in the rapidly advancing telecommunications field for example, electronically generated timing signals are becoming increasingly problematic as fundamental limits of integrated circuit frequency response are reached. However, the burgeoning field of optoelectronics offers a means of avoiding a strict dependence on electrical/electronic timing signal generation. Optoelectronics is predicated upon optical signal processing and inherently includes technologically satisfactory structures for confining and transmitting optical pulses over great distances. Since the speed of light has a well recognized constant value, given a particular transmission medium, a light pulse can be utilized to define a non-relative and non-relativistic methodology for measuring time as well as time intervals. A light pulse traveling at a constant velocity, traversing a known distance, in the same reference frame as an observer, provides a simple and inherently stable method for defining a time interval. Mechanical definition of a multiplicity of branching travel paths offers a straight forward way of constructing a timing generator characterized by timing trigger edges having native periodicities in the gigahertz and multi-gigahertz regime.